Persulfate is emerging as a cost-effective and viable oxidant for in situ chemical oxidation (ISCO) technology for the treatment of organic contaminants in groundwater, soils, and sediments. In situ soil and/or groundwater remediation typically involves injecting substances such as oxidants into the soil or groundwater to locations proximate to the contaminants or chemicals of concern (COC). The injected substances react with the contaminants or COCs in situ to eliminate them, to break them down into less harmful substances, and/or to otherwise neutralize them. One type of in situ remediation is referred to as surfactant enhanced in situ chemical oxidation (S-ISCO) remediation, disclosed in International Application No. PCT/US2007/007517, filed on Mar. 27, 2007, the entire contents of which are hereby incorporated by reference.
In comparison with permanganate, persulfate chemistry brings about a greater decrease in the soil oxidant demand (SOD) and also promotes the formation of free radicals (sulfate free radical SO4−. and hydroxyl free radical OH.), possibly ferryl (FeO2+)-EDTA complex, a number of free radicals of the contaminants during their oxidation and a family of free radicals known to exist associated with the hydroxyl radical induced chain reactions. In contrast, permanganate does not have a free radical pathway. Because of its relatively high stability under normal subsurface conditions, persulfate travels through the subsurface into targeted contaminant zones more effectively than does hydrogen peroxide associated with Fenton's Reagent and Modified Fenton's Chemistry. While direct oxidation of many organic chemicals in the subsurface by persulfate is possible by direct oxidation pathways alone, the greatest potential for persulfate is realized when it is activated to form free radical species.
The formation of free radical species in the subsurface from persulfate requires both persulfate and an activator to be present in the zone desired for treatment. Ideally, injected persulfate and activator solutions would be able to migrate substantial distances through the subsurface, providing a continuous and sustained level of free radical production throughout the contaminated location targeted for treatment.
Heat activation of persulfate in the subsurface has limited application because of the significant thermal energy required to evenly heat the subsurface. A classical Fenton-like Fe2+ activation of persulfate requires acidification of the subsurface, which in most cases is impractical, infeasible or undesirable because of the potential mobilization of heavy metals at low pH values and cost. Fe2+ activation of persulfate in acidic solutions promotes rapid production of free radicals followed by stalling of the activation due to oxidation of Fe2+ to Fe3+. Iron chelates have been used to slow the activation of persulfate, in order to increase the longevity of divalent metal cations and to provide a more sustained activation of persulfate. The sustained persulfate activation obtained using either Fe(II) or Fe(III) chelated with ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), L-ethylenediaminedisuccinic acid (EDDS), and nitrilotriacetic acid (NTA) results from progressive oxidation of the chelate. Once the chelate species is oxidized or penetrated by persulfate, its chelating capacity is reduced and the iron cation is available to activate persulfate.
From a practical standpoint, oxidation of the chelating ligand in persulfate systems is viewed as advantageous, as the destruction of the ligand may alleviate concerns about the introduction of another component into the subsurface environment and the potential for heavy metal mobility in the subsurface. The oxidation rates of chelates varies depending upon the chelate used and the pH of the system. The use of metal chelates to activate persulfate can potentially be applied in situations where in situ chemical oxidation is needed. However, a limitation of the iron chelates disclosed in the prior art is that most chelates are oxidized by persulfate and its free radical species resulting in limited transport in the subsurface. As a consequence, persulfate generally has a much longer life in the subsurface than iron chelates. Therefore, the distance that an iron chelate travels in the subsurface is generally much shorter than that for persulfate. This requires additional injection wells for the iron chelate and may result in persulfate that is not activated and therefore not able to degrade contaminants targeted for treatment. Further, it has been demonstrated that iron chelate activated persulfate is not as effective in destroying certain classes of contaminants, such as chloroethanes. When activation is required, such as in the case of persulfate and peroxide, the longevity of the activator should ideally be close to that of the oxidant. One factor that may limit the ability of iron-chelate activators to travel as far in the subsurface as persulfate is that iron-chelate complexes can be degraded rapidly by persulfate.
Recently, zero-valent iron (ZVI) has been used to activate hydrogen peroxide for pentachlorophenol (PCP) and methyl-tertiary-butyl ether (MTBE) destruction. The use of powdered ZVI to activate hydrogen peroxide has been shown to increase the rate and extent of compound destruction. Unfortunately, the very rapid reaction rates of ZVI and the rapid oxidation of ZVI with hydrogen peroxide limits the applicability of this technology in subsurface applications.
In addition, the development of nano-scale ZVI processes for injection into the subsurface has also gained great attention in the art recently. To date, nano-scale ZVI has been injected in an aqueous slurry, mixed with an organic guar material to provide structural integrity for emplacement into fractures or permeable reactive barriers (PRBs), or mixed with molasses or another type of biodegradable substrate to promote simultaneous physical, chemical and/or biological reduction processes. When organic guar is used with ZVI, typically an enzyme is added once the guar-ZVI has been emplaced to induce biodegradation of the guar, thus exposing the ZVI to contaminated groundwater.